Method and apparatus for detecting a change in capacitance of a capacitive proximity sensor

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No. 60/330,171, filed Oct. 17, 2001, the contents of which are incorporated by reference thereto, and

This application claims the benefit of U.S. provisional application No. 60/330,173, filed Oct. 17, 2001, the contents of which are incorporated by reference thereto.

This application further claims the benefit of U.S. provisional application No. 60/361,803, filed Mar. 5, 2002, the contents of which are incorporated by reference herein in their entirety.

BACKGROUND

The present disclosure relates generally to proximity detection systems and, more particularly, to a method and apparatus for detecting a change in capacitance of a capacitive proximity sensor.

Various systems have been devised for detecting obstacles in the path of a moveable panel such as an automotive power window, power sliding door or power hinged door. When an obstacle is detected, forward movement (e.g., closing) of the panel is interrupted and, optionally, the movement of the panel may be thereafter reversed (e.g., opened). These detection systems may generally be characterized as either “contacting” or “non-contacting”. In a contacting system, an obstacle is detected only after some form of physical contact occurs between the panel and the obstacle, and may include devices such as pneumatic/pressure sensitive strips, or possibly sensors responsive to changes in mechanical or electrical loading in the apparatus that moves the panel.

On the other hand, in a non-contacting system, an obstacle is detected before actual contact occurs. One specific type of non-contacting obstacle detection system employs the use of a capacitive element(s) as a proximity sensor(s). Capacitive proximity sensors may include one or more electrical conductors formed along the leading edge of a moveable panel, as well as a capacitance sensitive circuit (e.g., a bridge circuit or an oscillator) coupled to the conductor(s). An obstacle (e.g., a human hand) in proximity to the conductor(s) changes the capacitance of the sensor, which change is thereafter detected by the capacitive sensitive circuit.

Unfortunately, certain difficulties are inherent in creating a sensitive capacitive proximity system that can distinguish between changes in environmental conditions and an actual foreign object. A sufficiently sensitive capacitive proximity detection system should be responsive to relatively small changes in sensor capacitance. However, certain high frequency components used in conjunction with a sensitive device can result in unwanted radiated emissions to neighboring electronic components. In addition, any control module used for the processing of a detected capacitance change will generally be provided with a lower frequency integrated circuit(s), thus presenting a compatibility problem between the higher frequency sensor signal and the integrated circuit.

SUMMARY

The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for detecting a change in capacitance of a capacitive sensing element having a nominal capacitance value. In an exemplary embodiment, the method includes coupling the sensing element to a first oscillator, the first oscillator generating a first frequency dependent upon the capacitance value of the sensing element. The first frequency is compared to a reference frequency generated by a second oscillator. The change in capacitance from the nominal capacitance value is detected if the first frequency differs from said reference frequency by a determined frequency value.

In a preferred embodiment, the first oscillator and the second oscillator are configured such that the first frequency and the reference frequency are offset from one another by a fixed offset value when the capacitance of the capacitive sensing element is equal to the nominal capacitance value. In addition, the first frequency and the reference frequency are inputted into a mixer. An output of the mixer is then inputted into a low pass filter, wherein an output of the low pass filter is an intermediate frequency (IF) that corresponds to the difference between the first frequency and the reference frequency. The intermediate frequency (IF) is preferably in the range of about 0 to about 15 MHz, while the first frequency and reference frequency are at least 900 MHz.

In still a further embodiment, the intermediate frequency is transmitted to a control module where the intermediate frequency is then compared to the fixed offset value. Thereby, an increase in capacitance from the nominal capacitance value is detected if the intermediate frequency exceeds the fixed offset value.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:

FIG. 1 is a schematic diagram of a non-contact obstacle detection system, including one or more capacitive sensing elements and associated high sensitivity capacitance detection circuits, in accordance with an embodiment of the invention;

FIG. 2 is a block diagram of an exemplary high sensitivity capacitance detector (HSCD) along with the associated control module circuitry connected thereto;

FIG. <b>3</b>(<i>a</i>) is a schematic diagram illustrating one possible circuit implementation of the HSCD shown in FIG. 2; and

FIG. <b>3</b>(<i>b</i>) is a schematic diagram illustrating one possible circuit implementation of the associated control module circuitry shown in FIG. <b>2</b>.

DETAILED DESCRIPTION

Disclosed herein is a method and apparatus for detecting a relatively small change in the capacitance of a capacitive sensing element, such as may be found in a non-contact obstacle detection system. Although the following disclosure is described in the context of an obstacle detection system for use in conjunction with a power door or gate of a motor vehicle, it will be understood that present invention embodiments are equally applicable to any proximity detection system and, more generally, to any system using one or more capacitive sensing elements.

Referring initially to FIG. 1, there is shown a schematic diagram of a non-contact obstacle detection system <b>10</b>, which could be implemented, for example, in conjunction with a motor vehicle power door assembly. The obstacle detection system <b>10</b> may include one or more capacitive sensing elements <b>12</b>, each configured to have a desired nominal capacitance value. Each individual capacitive sensing element <b>12</b> further includes a flexible capacitive strip <b>14</b> (as highlighted in insert “A”) having a high sensitivity capacitance detector (HSCD) circuit <b>16</b> integrated therein. The HSCD circuit <b>16</b> is symbolized in insert “B”.

The sensing elements <b>12</b> are each coupled to a central control module <b>18</b> through lines <b>20</b>. As is described in greater detail hereinafter, the lines <b>20</b> preferably include a twisted pair of conductors or, alternatively, a length of coaxial cable. In either case, the lines <b>20</b> provide a means of supplying a direct current (DC) power signal from the central control module <b>18</b> to the HSCD circuit <b>16</b>. Furthermore, the central control module <b>18</b> receives an output signal from each of the sensing elements <b>12</b> through lines <b>20</b>, and determines whether there is a detected change in capacitance of any of the sensing elements <b>12</b>. In the event that a foreign object is in proximity to one or more sensing elements <b>12</b> during a closing operation of a power door or panel, the central control module <b>18</b> will generate a command signal to stop the closing operation. This may be accomplished, in one embodiment, by coupling the central control module <b>18</b> to a power door control module <b>22</b> through a data line <b>24</b>.

Additional details regarding the obstacle detection system <b>10</b> may be found in U.S. patent applications, Ser. No. 10/142,643, entitled “Flexible Capacitive Strip for Use in a Non-Contact Obstacle Detection System”, Ser. No. 10/142,641, entitled “Method and Apparatus for Detecting a Change in Capacitance of a Capacitive Proximity Sensor”, Ser. No. 10/142,680, entitled “Capacitive Sensor Assembly for Use in a Non-Contact Obstacle Detection System”, each of which were filed on May 10, 2002, the contents of which are incorporated herein by reference. Generally speaking, the central control module <b>18</b> will preferably include computer hardware and associated software therein (represented symbolically in insert “C”) for comparing output signals from the HSCD circuits <b>16</b> to nominal values stored in lookup/profile tables.

As shown in insert “A”, the flexible capacitive strip <b>14</b> (in one possible embodiment) may be formed by extruding an insulating, flexible material <b>26</b> (such as santoprene rubber) around a flat conductor <b>28</b>. The conductor <b>28</b> serves as one capacitor electrode of the capacitive sensing element <b>12</b>. The conductor <b>28</b> is further designed to be substantially parallel to a surface (not shown), along the entire length of the strip <b>14</b>, when the strip <b>14</b> is affixed to the surface. In one embodiment, the conductor <b>28</b> serves as one capacitor electrode, while the surface (e.g., a metallic vehicle body) serves as the other electrode. An air cavity <b>30</b> within the strip <b>14</b> serves, in one aspect, as a dielectric material between the capacitor electrodes. Alternatively, the sensing element <b>12</b> may be designed with two conductors molded internally therein. Additional details regarding the capacitive sensing element <b>12</b> may be found in U.S. Patent applications, Ser. No. 10/142,643, entitled “Flexible Capacitive Strip for Use in a Non-Contact Obstacle Detection System”, Ser. No. 10/142,641, entitled “Method and Apparatus for Detecting a Change in Capacitance of a Capacitive Proximity Sensor”, Ser. No. 10/142,680, entitled “Capacitive Sensor Assembly for Use in a Non-Contact Obstacle Detection System”, each of which were filed on May 10, 2002, the contents of which are incorporated herein by reference.

Referring now to the block diagram of FIG. 2, the HSCD <b>16</b> includes a fixed (local) oscillator <b>40</b>, a variable (RF) oscillator <b>42</b>, a mixer <b>44</b>, a low pass filter <b>46</b>, gain stage <b>48</b>, and a bias “T” arrangement <b>50</b>. The fixed oscillator <b>40</b> produces a frequency output that is stable in nature. By way of example, the frequency of the fixed oscillator <b>40</b> may be set at a fixed frequency, f_LO, of about 925 MHz. However, it can be set at any desired frequency. The variable oscillator <b>42</b> is coupled to the capacitive strip <b>14</b> of a capacitive sensing element <b>12</b> as described above, such that the variable oscillator <b>42</b> generates an output frequency that is dependent upon the capacitance value of the capacitive strip <b>14</b>. In a preferred embodiment, the variable oscillator <b>42</b> is set up to generate a frequency, f_RF, of about 922 MHz (or, in other words, at a fixed offset of about 3 MHz from the fixed oscillator) in a steady state environment. A “steady state” environment refers to a condition wherein there is no foreign object in proximity to a given sensing element <b>12</b>. In such a situation, the capacitance value of the strip <b>14</b> remains at its nominal value as determined by its physical parameters, and ΔC=0.

If a hand or other foreign object approaches the sensor strip <b>14</b>, the capacitance value thereof will increase such that ΔC≠0. As a result, the output frequency, f_RF, of the variable oscillator <b>42</b> is also changed such that Δf_RFis proportional to ΔC. More specifically, an increase in the capacitance of the sensor strip <b>14</b> will cause the output frequency, f_RF, of the variable oscillator <b>42</b> to decrease.

Both the fixed oscillator output frequency and the variable oscillator output frequency are fed through return loss attenuators <b>43</b><i>a </i>and <b>43</b><i>b</i>, respectively, and inputted into mixer <b>44</b> where the two signals are both subtracted and added together. As is well known in the art, mixers are used in RF communication technology to shift a baseband information signal to a frequency or frequencies suitable for electromagnetic propagation to the desired destination. In this case, the frequency to be shifted is the variable frequency f_RF. When mixed with the output of the fixed (i.e., local) oscillator <b>40</b>, the result is two outputs at the intermediate frequency (IF) port of the mixer <b>44</b>. These IF outputs may be represented as f_LO+f_RFand f_LO−f_RF. In a steady state mode, therefore, the resulting mixer output frequencies are about 3 MHz and 1847 MHz.

If the desired IF signal is higher than the RF signal, the mixer is considered an upconverter; if the desired IF signal is lower than the RF signal, then the mixer is considered a downconverter. As is described hereinafter, the desired IF component for the present HSCD <b>16</b> is the frequency difference between the local oscillator <b>40</b> and the fixed oscillator <b>42</b>. Thus, frequency down conversion is applied by inputting the output of the mixer <b>44</b> through the low pass filter <b>46</b> in order to pass through the difference of the two frequencies. The low pass filter <b>46</b> is preferably designed such that a capacitance change seen at a sensing element <b>12</b> (and resulting frequency change of the variable oscillator <b>42</b>), caused by the presence of a foreign object, is recognized and detected in the form of a change (i.e., increase) in f_LO−f_RF.

The output of the low pass filter <b>46</b> is then amplified by gain stage <b>48</b> before being transmitted to the control module <b>18</b> for processing. As will be noted later, the low pass filter <b>46</b> can also be configured to be part of the feedback network of gain stage <b>48</b>. The bias “T” arrangement <b>50</b> is incorporated to reduce the number of wiring connections used to couple the control module to the HSCD. As is known in the art, a bias “T” is used to couple a DC voltage onto the same transmission line used for AC (RF) signals, or to detect/remove the DC component of a composite signal. Thus, if a bias “T” is used, a two-wire twisted pair conductor may be used for lines <b>20</b> to couple the control module <b>18</b> to the HSCD <b>16</b>. Alternatively, a coaxial cable may be used.

In a simple form, a bias “T” may include an inductive choke or, for frequencies exceeding 1 GHz, specifically tuned elements on a circuit board. Additional information on bias “T” arrangements may be found in U.S. Pat. No. 6,229,408 to Jovanovich, et al. It will be appreciated that, if multiple sensing elements <b>12</b> are used in system <b>10</b>, then the central control module <b>18</b> will include a multiple bias “T” arrangement. In the present embodiment, the AC signal output from gain stage <b>48</b> travels through the bias “T” <b>50</b> to the control module <b>18</b>, while a 5-volt (for example) DC signal is sent from the control module <b>18</b> to the HSCD <b>16</b> to provide power thereto.

As shown in FIG. 2, the control module <b>18</b> farther includes a front-end electronics area <b>19</b> having a corresponding bias “T” <b>21</b>. An internal power supply <b>23</b> provides a regulated, +5V output voltage for use by the HSCD <b>16</b>. On the receiving side, bias “T” <b>21</b> also strips off the incoming IF signal from the HSDC <b>16</b>, so that the IF signal may be passed through a Schmidt trigger <b>25</b> and then on to digital processing by software within the control module <b>18</b>.

Referring now to FIG. <b>3</b>(<i>a</i>), there is shown one possible circuit embodiment that realizes the block diagram functions of the HSCD <b>16</b> illustrated in FIG. <b>2</b>. As is shown, the bias “T” <b>50</b> includes coupling capacitor C<b>9</b> and inductor L<b>1</b>. Capacitor C<b>9</b> couples the output IF signal from low pass filter <b>46</b> and gain stage <b>48</b> onto the signal line, while the inductor L<b>1</b> provides a high AC impedance so as to prevent the IF signal from being fed back onto the +5V DC bus <b>60</b>. An exemplary value for L<b>1</b> is about 220 microhenries (μH), while C<b>9</b> may be chosen to be about 0.047 microfarads (μF).

A plurality of filter capacitors C<b>1</b>, C<b>2</b>, C<b>3</b> and C<b>4</b> are used to reduce any ripple effects on the DC bus <b>60</b> that could be generated by the RF oscillator <b>42</b> and the local oscillator <b>40</b>. Particularly, C<b>2</b> and C<b>3</b> also serve to isolate one oscillator from the other. The capacitance values of C<b>1</b> through C<b>4</b> are relatively small (e.g., 0.1 μF) as compared to that of capacitor C<b>8</b> (e.g., 3.3 μF), C<b>8</b> being used to store the DC energy supplied to the HSCD.

FIG. <b>3</b>(<i>a</i>) further schematically illustrates the capacitive strip <b>14</b> coupled to the RF oscillator <b>42</b>. Although not shown, a series capacitor is preferably coupled between the capacitive strip <b>14</b> and the RF oscillator <b>42</b>, thereby forming a capacitive divider, in order to reduce the sensitivity of the circuit and to keep radiated emissions in the FCC approved band. In addition, resistors R<b>1</b> and R<b>4</b> (e.g., 2.1 kΩ and 1.2 kΩ, respectively) are configured as a voltage divider to provide a trim point for the RF oscillator <b>42</b>.

The local oscillator <b>40</b> is preferably buffered down with a bias loading so as to maintain a steady state, 3 MHz offset from the RF oscillator <b>42</b>. Accordingly, an external capacitor C<b>5</b> (having an exemplary capacitance of about 30 pF) is coupled to the local oscillator <b>40</b>. Resistors R<b>2</b> and R<b>4</b> (e.g., 2.1 kΩ and 560 Ω, respectively) are configured as a voltage divider to provide a trim point for local oscillator <b>40</b>.

Although the obstacle detection system <b>10</b> utilizes environmental compensation software in its control module <b>18</b>, an additional measure of compensation for variation in operating conditions (e.g., temperature changes) is provided by using the same type of oscillator for both the RF oscillator <b>42</b> and the local oscillator <b>40</b>. In so doing, the effect on the output frequency of the local oscillator <b>40</b> (for example, as the result of a temperature change) will not affect the frequency difference between the RF oscillator <b>42</b>, since the temperature change should have the same effect on the output frequency of the RF oscillator <b>42</b>.

The low pass filter <b>46</b> and gain stage <b>48</b> are shown in FIG. <b>3</b>(<i>a</i>) as an integrated element. A first order, low pass filter is configured by coupling capacitor C<b>6</b> to the output of mixer <b>44</b>. The capacitance value of C<b>6</b> may be chosen to be about 30 nanofarads (nF) to provide a 3 dB signal attenuation at around 10 MHz. The gain stage <b>48</b> includes an operational amplifier <b>62</b>, with associated bias resistors R<b>4</b> and R<b>5</b>, feedback resistors R<b>6</b>, R<b>7</b> and R<b>8</b>, and feedback filter capacitor C<b>7</b>.

Finally, FIG. <b>3</b>(<i>b</i>) is a schematic diagram illustrating one possible circuit implementation of the associated control module circuitry <b>19</b> shown in FIG. <b>2</b>. The corresponding bias “T” <b>21</b> therein includes inductor L<b>2</b> and capacitor C<b>10</b>. A pair of zener diodes Z<b>1</b>, Z<b>2</b> provide voltage regulation for the output of power supply <b>23</b>, as well as the Schmidt trigger <b>25</b>. Capacitor C<b>11</b>, C<b>12</b> and C<b>13</b> further provide filtering for the power supply <b>23</b>, which illustrates (by way of example) a +12 VDC input voltage, common in automotive systems.

It should be understood that the circuit shown in FIG. <b>3</b>(<i>a</i>) represents but one exemplary implementation of the HSCD, and those skilled in the art will recognize that several alternative circuit configurations are possible and may be configured to fall within the scope of the present invention embodiments.

As will be appreciated from the foregoing description, the HSCD method and apparatus takes advantage of frequency down conversion to allow for sufficient sensitivity to detect small changes in capacitance, while providing for the high frequency components (local oscillator <b>40</b>, variable oscillator <b>42</b>) to be localized at the sensing elements. In other words, a change in sensor strip capacitance, initially represented by a small frequency change in the output of the variable oscillator <b>42</b>, is down converted to an intermediate frequency (IF) that is more suitable for use by the remaining portion of the overall system. Accordingly, the frequency down conversion technique keeps the 900 MHz signals contained within the HSCD <b>16</b>, while the difference signal f_LO−f_RFis the only frequency transmitted to another area of the vehicle. Since the difference signal ranges only from about 0 to about 15 MHz, it is possible (with the bias “T”) to use a twisted pair for the signal transmission to the central control module <b>18</b>, thereby providing a more cost effective alternative to a coaxial cable and connector system. Of course and in an alternative embodiment, the circuit or system can be adapted to operate outside the aforementioned range.

It should also be understood, however, that although a preferred frequency range of operation has been discussed, the HSCD will operate at any frequency. The embodiments discussed operate in the 902-928 Mhz ISM band, set aside by the FCC for field disturbance sensors. This band is wide enough to accommodate the swing of the RF oscillator. At lower frequencies, however, the swing may exceed the specified FCC band limits. Moreover, the <b>900</b> MHz band allows for a higher radiated power level than at lower frequencies. While the sensitivity of the HSCD is not dependent upon the oscillator power level, the FCC does regulate the maximum radiated power level.

Another benefit of the present design is that a clean digital signal is transmitted to the central control module <b>18</b>. Smooth edges due to a sine wave limits 2^ndand 3^rdorder harmonics to acceptable levels to meet the radiated emission requirements included the FCC regulations and applicable OEM specifications. In addition, variances in the power supply parameters (ripple or voltage drops) do not change the output signal frequency in the present design, since both oscillators will shift up or down in frequency as a result of any the above mentioned conditions. In other words, system sensitivity is not dependent on specific the power levels present thereon, provided the oscillators remain stable.

Still a further benefit of the present HSCD design is that the proximity detection system is self-adjusting for environmental conditions. Since the low pass filter <b>46</b> is designed to have a bandwidth of 15 MHz (although a wider bandwidth may be chosen if desired) and the steady state frequency difference, f_LO−f_RF, is set up at approximately 3 MHz, a 2 MHz change (decrease) in f_RFis sufficient to determine that an obstacle is in proximity to the sensor element. Accordingly, the dynamic range of the HSCD may be increased and f_LO−f_RFmay be set to allow for environmental changes such as temperature change, mud splattering, light rain, and other conditions.

It should also be pointed out that the sensor strip design itself is of particular concern with regard to the functioning of the HSCD <b>16</b>. The steady state capacitance value of the strip <b>14</b> should not be too large so as to swamp out the RF oscillator. If the capacitance value is too large, then the RF oscillator <b>42</b> could be pulled down and prevented from operating at the proper frequency, thereby becoming unstable. Thus, the impedance of the strip <b>14</b> is preferably chosen from about 50 ohms (Ω) to about 200 Ω. An impedance of about 200 Ω provides more device sensitivity due to a lower capacitance value, but the output radiated power is increased. At 50 Ω, the radiated power is reduced, but the device sensitivity decreases due to a larger steady state capacitance, thereby limiting the operating frequency range. Accordingly, a strip <b>14</b> having a characteristic impedance of about 100 Ω provides a good compromise for sensitivity and radiated emission concerns.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.